If you liked
Gantz
|
...then you might like
Kamisama no Iutoori 2
|
These are both so similar I wouldn't be surprised if one was inspired by the other. Both involve having people play a game where they must survive, people die, romances bloom. Read as they do their best to survive and overcome the mastermind which both allegedly come from outer space. They are both long giving you space to really become immersed in the story and watch relations develop, there are badass scenes and desperate times.
If you liked
Gantz
|
...then you might like
Imawa no Kuni no Alice
|
Imawa no Kuni no Alice has people participate in a game where they are fighting for their life. Both have great action scenes and lots of mind breaking content. (Imawa no Kuni no Alice is incomplete so I'd suggest the manga instead.)
If you liked
Naruto
|
...then you might like
Dragon Ball
|
Naruto and Dragon are both highly recognized shounen anime. Dragon Ball shows the life of Goku as a child where he trains to get stronger, just as Naruto does in this series. Each of them are great prequels to serve as a stepping stone towards their even more action-packed sequels.